Direct device-to-device or peer-to-peer communication can be exploited in cellular networks to improve the overall network capacity as well as to mitigate coverage holes for user terminals (e.g. User Equipment's (UE)) or other network connected devices (e.g. Machine to Machine (M2M) devices) that do not have network coverage. For simplicity, UE will be used throughout this document as meaning any type of wireless network connected device. The performance and advantages of such D2D communications is currently an active topic of investigation in the Third Generation Partnership Project (3GPP) Radio Access Network (RAN).
The D2D communication may be bi-directional communication where both devices receive and transmit in the same or different resources. However, a D2D communication scenario may also comprise that one of the devices transmits and the other one receives the signals. There may also exist a point-to-multipoint (e.g. multicast, broadcast) scenario in which case a plurality of devices receive signals from the same transmitting device. This scenario is particularly useful for emergency services or public safety operations to spread vital information to several devices in an affected area. The term D2D communication and D2D operation are interchangeably used herein.
Devices typically operate under the supervision of a radio access network with radio access nodes (e.g. base station). But in some scenarios the devices themselves establish direct communication constituting the radio access network, without the intervention of the network infrastructure.
In cellular network assisted D2D communications (or simply network assisted D2D communications), UEs in the vicinity of each other can establish a direct radio link (e.g. referred to as a D2D bearer). While UEs communicate over the D2D “direct” bearer, they also maintain a cellular connection with their respective serving base station (e.g. evolved NodeB (eNB)). This direct link is interchangeably called as NetWork (NW) link, D2D-NW link etc. The NW link is used for e.g. resource assignment for D2D communication, maintenance of radio link quality of D2D communication link etc.
Three relevant coverage scenarios for D2D communication are shown in FIG. 1. The left most scenario illustrates partial coverage, the middle scenario illustrates in coverage and the right most scenario illustrates out of coverage.                In coverage: This scenario is the middle scenario in FIG. 1. In this scenario, all D2D UEs 101 communicating are under the network node coverage. This means that the D2D UEs 101 can receive signals from and/or transmit signals to at least one network node 102. In this case, the D2D UE 101 can maintain a communication link with the network node 102. The network node 102 in turn can ensure that the D2D communication does not cause unnecessary interference. In coverage is also interchangeably called In-Network (IN) coverage. In FIG. 1, the network coverage is illustrated with a circle. There are two D2D UEs 101 communicating in FIG. 1, e.g. UE A and UE B. Since both UE A and UE B 101 are within the circle, they are both in coverage of the network node 102. The arrow between the D2D UEs 101 illustrates the D2D communication. The network node 102 which the D2D UEs 101 can receive signals from and/or transit signals to is exemplified with a base station in FIG. 1.        Out of coverage: This scenario is the right most scenario in FIG. 1. In this scenario, D2D UEs 101 communicating with each other are not under network node 102 coverage. This means that the D2D UEs 101 cannot receive signals from and/or transmit signals to any network node 102. Typically, the lack of coverage is due to complete absence of the network coverage in the vicinity of the D2D UEs 101. However, the lack of coverage may also be due to insufficient resources in the network nodes 102 to serve or manage the D2D UEs 101. Therefore in this scenario the network cannot provide any assistance to the D2D UEs 101. The out of coverage is also interchangeably called Out-Of-Network (OON) coverage. In FIG. 1, the out of coverage scenario is illustrated with two D2D UEs 101, e.g. UE A and UE B. The absence of any network node 102 in FIG. 1, illustrates that the D2D UEs 101 are out of coverage from any network node 102. The arrow between the D2D UEs 101 illustrates the D2D communication.        Partial coverage: This scenario is the left most scenario in FIG. 1. In this scenario at least one D2D UE 101 communicating is under the network coverage, and at least one D2D UE 101 communicating is not under the network coverage. As mentioned above the D2D UE 101 not being under the network coverage can be due to lack of any network node 102 in its vicinity or due to insufficient resources in any of the network nodes 102 in its vicinity. The partial coverage is also interchangeably called as Partial-Network (PN) coverage. The network node 102 coverage is illustrated with a circle in FIG. 1. There are two D2D UEs 101 seen in FIG. 1. UE A 101 is seen to be outside the circle, i.e. it is out of coverage, and UE B 101 is inside the circle, i.e. it under coverage. The arrow between the D2D UEs 101 illustrates the D2D communication. The network node 102 which the D2D UEs 101 can receive signals from and/or transit signals to is exemplified with a base station in FIG. 1.        
The emissions outside the Band Width (BW) or frequency band of the D2D UE 101 are often termed as out of band emissions or unwanted emissions. The major Out-Of-Band (OOB) and spurious emission requirements are typically specified by the standard bodies and eventually enforced by the regulators in different countries and regions for both the D2D UE 101 and the network nodes 102 (e.g. a base station). Examples of the OOB emissions are Adjacent Channel Leakage Ratio (ACLR) and Spectrum Emission Mask (SEM). Typically, these requirements ensure that the emission levels outside the transmitter channel bandwidth or operating band remain several tens of decibels (dB) below the transmitted signal.
For the D2D UE 101, the conservation of its battery power is very critical. This requires that the D2D UE 101 has an efficient Power Amplifier (PA). The PA is therefore typically designed for certain typical operating points or configurations or set of parameter settings e.g. modulation type, number of active physical channels (e.g. resource blocks in Evolved-Universal Terrestrial Radio Access (E-UTRA) or number of Code Division Multiple Access (CDMA) channelization codes code/spreading factor in Universal Terrestrial Radio Access (UTRA)). To ensure that the D2D UE 101 fulfills OOB/spurious requirements for all allowed UpLink (UL) transmission configurations, the D2D UE 101 is allowed to reduce its maximum UL transmission power in some scenarios. This is called Maximum Power Reduction (MPR) or UE power back-off in some literature. For instance, a D2D UE 101 with maximum transmit power of 24 dBm power class may reduce its maximum power from 24 dBm to 23 or 22 dBm depending upon the configuration.
In E-UTRA, an Additional-Maximum Power Reduction (A-MPR) for the D2D UE transmitter has also been specified in addition to the normal MPR. The A-MPR can vary between different cells, operating frequency bands and more specifically between cells deployed in different location areas or regions. In particular, the A-MPR may be applied by the UE in order to meet the additional emission requirements imposed by regional regulatory organizations. A-MPR is an optional feature which is used by the network when needed depending upon the co-existence scenario. The A-MPR defines the D2D UE maximum output power reduction (on top of the normal MPR) needed to fulfill certain emission requirements by accounting for factors such as: bandwidth, frequency band or resource block allocation. The A-MPR is therefore controlled by the network node 102 by signaling to the D2D UE 101 a parameter called Network Signaling (NS) parameter. For example, NS_01 and NS_02 correspond to different levels of pre-defined A-MPRs.
Even in case of network assisted D2D communication, the network may not fully manage the interference. Therefore, there exists a potential for D2D communications to cause interference to both serving cellular networks as well as in legacy co-located networks or co-existing networks in the same geographical region.
In Long Term Evolution (LTE), potential D2D interference can be intra-frequency co-channel interference—i.e. collisions between transmitted Resource Blocks (RBs) within the system bandwidth, as well as interference from in-band emissions from the transmitting RBs within the system bandwidth into adjacent RBs to those RBs being employed for the desired transmission. In addition, D2D communications can result in inter-device and intra-device interference across a number of channels in LTE including for example Physical Uplink Control Channel (PUCCH) and Physical Uplink Shared Channel (PUSCH) channels. The D2D communication typically takes place over LTE uplink channels such as PUCCH/PUSCH or similar channels. These are described below:
Inter-Device Interference
The inter-device interference scenario is explained with an example comprising two devices, A and B, which communicate via D2D communication in given subframes 2, 3 and 4 on the UL e.g. on UL spectrum in Frequency Division Duplex (FDD). In these subframes, the device B receives information from device A in a first set of RBs. Also during these subframes the device C transmits to an eNodeB, in UL resources in the same system bandwidth as B is receiving D2D communication from A, but in a second set of RBs. In this example, the second set of resources is a PUCCH transmission in subframe 2 and a PUSCH transmission in subframe 3.
Due to in-band emissions, device C will create a “high interference” area where B is possibly unable to decode data from A. This “high interference” area will be a function of:                Device C's transmit output power.        The path loss from device C to device B.        Device C's RB allocation.        The receive power level of device B and the D2D RB allocation.        Device C's in-band emission levels at the frequency of the D2D RB allocation.        
Such inter-device interference scenarios can clearly occur in both partial and full coverage scenarios. It is also possible that it could occur in “no-coverage” scenarios if for example devices A and B are both out of coverage and device C is within coverage but close to the edge of coverage and close to devices A and B, such that it can still create an exclusion zone for these devices.
Based on the D2D Work Item Description (WID) and the current proposals under discussion in RANI , D2D transmissions can be broadly classified into discovery or communications transmissions. Since PUCCH transmissions in general are pre-assigned with a fixed periodicity, the PUCCH transmissions could potentially impact both the discovery and the communications phases of D2D. However, with regard to PUSCH transmissions, the PUSCH transmissions of device C could be scheduled to avoid the discovery phase of the D2D transmissions, but likely not the D2D transmissions during the communications phase. The interference zone due to the in-band emissions for these inter-device scenarios can be quite large, potentially in the order of 10 s or 100 s of meters.
Intra-Device Interference
Intra-Device interference is related to the case when a device A is transmitting simultaneously both to a nearby device B using D2D communication in a first set of resource blocks, and transmitting to a network node 102 using a second set of RBs. An example scenario for this would be when a device A transmits a beacon signal (or pilot signal) and simultaneously transmits a PUCCH to the network node 102, but other scenarios may also exist. Note that the intra-device interference will be limited to full and partial coverage scenarios.
As noted previously, there also exists the potential for D2D communications to cause interference to both serving cellular networks as well as legacy networks especially which are co-located with the serving cellular networks. The interference may also be caused to the networks, which co-exist in the same geographical areas where D2D UEs 101 operate.
Scenario 1 illustrated in FIG. 2a and scenario 2 illustrated in FIG. 2b below cover the use cases in which regular LTE UL transmissions act as an aggressor or interferer to D2D transmissions both for the victim D2D UE 101 being out-of-network coverage (scenario 1, FIG. 2a) and in-network or partial coverage (scenario 2, FIG. 2b). The LTE transmissions can be for both FDD and Time Division Duplexing (TDD) implementations. In FIGS. 2a and 2b, the dot indicates the transmission interfered with, the solid arrow indicates the interferer, the dashed arrow indicates the desired D2D transmission and the dotted arrow indicates the desired LTE transmission. The network node 102 is exemplified with an eNB 102 in FIGS. 2a and 2b. 
As mentioned above, scenario 1 in FIG. 2a covers LTE FDD and LTE TDD. Four UE's 101 are illustrated in FIG. 2a, i.e. A, B, C and D. Two of the UEs 101, e.g. B and C, are in coverage of the eNB 102 (they are illustrated as being inside the circle which represents the network coverage). Two of the UEs 101, e.g. A and D are out of coverage of the eNB 102 (they are illustrated as being outside the circle which represents the network coverage). The aggressor (also referred to as interferer) is UE C 101 which performs an LTE UL transmission to the eNB 102, indicated with the dotted arrow in FIG. 2a. UE B 101, UE C 101 and UE D 101 are interferers, indicated with solid arrows. The victims are the in-coverage receiver UE B and the out-of-coverage receiver UE D. The desired D2D transmission between the UE A 101 and the UE B 101 and UE D 101 are interfered, indicated with a dot on the dashed arrows.
As mentioned above, scenario 2 in FIG. 2b covers LTE FDD and LTE TDD. Scenario 2 is similar to scenario 1, except that the D2D transmission is in coverage in scenario 2. Four UE's 101 are also illustrated in FIG. 2b, i.e. A, B. C and D. Two eNBs 102 are illustrated in FIG. 2b, eNB1 and eNB 2. Each of the eNBs 102 has its own coverage area indicated with a circle. UE A 101 and UE B 101 are in coverage of eNB1 102 and UE C 101 is in coverage of eNB2 102. UE D 101 does not have any network coverage.
UE C 101 is the aggressor which have an LTE UL transmission with eNB2 102, indicated with a dotted arrow in FIG. 2a. The D2D victims are the UE B 101 which is in coverage of eNB1 and UE D 101 which is out of coverage. UE C 101 is an interferer, indicated with solid arrows. UE B 101 and UE D 101 are interfered by UE C 101. The desired D2D transmission between the UE A 101 and the UE B and UE D are interfered, indicated with a dot on the dashed arrows.
Scenario 3 illustrated in FIG. 3a and scenario 4 illustrated in FIG. 3b cover the use cases in which a D2D transmission acts as an aggressor or interferer to LTE transmissions on the DL (i.e. LTE DL is the victim) for the D2D UE 101 being out-of-network coverage (scenario 3) and in-network or partial coverage (scenario 4). Note that these interference scenarios can only occur when the LTE network is operating in TDD duplex mode and the D2D transmission is not synchronized to the LTE network. For an FDD LTE network, since the D2D transmissions are on the UL, no co-channel interference will occur on the FDD DL channel, however interference to co-located co-existing networks can occur. In FIGS. 3a and 3b, the dot indicates the transmission interfered with, the solid arrow indicates the interferer, the dashed arrow indicates the desired D2D transmission and the dotted arrow indicates the desired LTE transmission. The network node 102 is exemplified with an eNB in FIGS. 3a and 3b.
As mentioned above, scenario 3 in FIG. 3a covers only LTE TDD. FIG. 3a illustrates three UEs 101, e.g. A, B and C. UE A 101 is in coverage of the eNB 102 (indicated by that UE A 101 is inside the circle representing the coverage area of eNB 102). UE B and C 101 are out of coverage from eNB 102 (indicated by that they are outside the circle representing the coverage area of eNB 102). The aggressor (also referred to as the interferer) is UE B 101 which is the D2D transmitter that is out of coverage. The victim is UE A 101 receiving LTE DL data from the eNB 102, indicated with the dotted arrow in FIG. 3a. Note that there is no synchronization between UE B 101 and eNB 102. The UE B 101 transmitting might cause interference to UE A 101.
As mentioned above, scenario 4 in FIG. 3b covers only LTE TDD. Scenario 4 is similar to scenario 3, except that the interferer is in coverage in scenario 4. FIG. 3b illustrates three UEs 101 where UE A and B 101 are in coverage of eNB1 102 and UE C is in coverage of eNB2 102. The UE B 101 is the interferer and the D2D transmitter is in coverage. UE C 101 is the victim and receives LTE DL data in another cell. Note that there is no synchronization between the UE B 101 and the eNB1 102. The UE B 101 transmitting might cause interference to the UE A 101.
The interfering situation becomes worse when D2D UEs 101 are in partial network coverage or even worse when they are completely out of network coverage. This may lead to the following problems:                The performance may be severely degraded.        The D2D communication may not be sustained.        Regulatory requirements on radio emissions may not be met by the D2D UE 101.        
Typically, D2D UEs 101 involved in D2D operation also maintain communication links with the network node 102. This communication link (aka network link or D2D UE-network link) enables the D2D UEs 101 to efficiently maintain their D2D operation. However, the network link may be fully or partially lost due to various reasons such as network disruption, unavailability of resources etc. This in turn will either degrade or terminate the D2D operation.